3. Vaccines: challenges and possibilities Flashcards
Different types of vaccines?
Live attenuated virus: Polio “sabin”, Measles, Rubella, Munps, Varicella, Yellow Fever
Inactivated virus: Polio “Salk”, Tick Borne Enceph.
Protein subunit: HepB, HPV (recombinant protein/VLP), seasonal Flu (“split” virus)
Plasmid DNA
Recombinant viruses
When is the greatest risk of transmission for HIV?
During acute infection
Less virus = less transmission risk
What is the HIV entry process?
Upon HIV binding to the primary receptor CD4, the envelope glycoproteins undergo conformational changes that expose the CoR binding site on gp120.
Further conformational changes are induced after CoR interactions, which allows gp41 to insert its fusion peptide into the target cell membrane to promote virus-cell fusion.
How can recombinant proteins provide opportunities to tailor vaccine responses?
- Individual Ags can be chosen
- Ags can be modified through protein engineering to change their properties in favourable ways
- Ags can be made as soluble subunit vaccines, as virus-like particles (eg. HPV) or as fusion proteins
- Well characterised and highly pure Ag preparations can be made
- Coadministration with an adjuvant usually required
What HIV vaccine clinical trials have been completed and what was their result?
VAX04 (Vaxgen): recombinant monomeric Env protein in Alum
*no protection observed
STEP: Recombinant adenovirus type 5 vector with HIV-1 T cell Ags (no envelope)
*no protection observed, possible enhancement observed
RV144: recombinant poxvirus vector followed by a boost with recombinant Env protein
*modest level of protection observed - 20% efficacy
Difference between HIV and Flu surface antigens?
HIV: a very large number of genetic variants circulate at the same time
*very mutagenic and difficult to predict its changes
Flu: a predominate viral variant circulates at a given time
*once virus is in you, it doesn’t change so immune response can develop
What are the HIV escape mechanisms? (Think HIV Env.)
- contains regions that tolerate an extreme degree of variation (V1-V5)
- has an extensive glycan shield
- has evolved to have the variable (immunodominant) and glycans (immunosilent) regions exposed on the outside of the Env trimer
How does high mutation rate aid in HIV-1 immune evasion?
Mutations introduced during viral replication change immune determinants on the virus, rendering existing (and previous) immune responses irrelevant.
Mutations are a feature of low fedelity of HIV RNA polymerase in replication.
How does the unstable association between gp120 and gp41 aid in HIV-1 immune evasion?
The unstable association results in shedding of gp120 and exposure of irrelevant (immunogenic) Ab epitopes.
Immune responses do not occur equally to all Ags, some dominate at the expense of others. Increasing the number of irrelevant targets increases the likelihood of irrelevant Abs dominating the response.
How do N-linked glycans aid in HIV-1 immune evasion?
Each gp120 molecule carries between 20-30 N-linked glycans, which together result in an efficient shield of antigenic sites on the trimeric spike.
Difficult to make ‘good’ neutralising Abs against carbohydrates.
Also coating the virus in sugar shields the region of HIV where neutralising Abs should bind.
How does access to the co-receptor binding site on gp120 affect HIV-1 immune evasion?
The highly conserved co-receptor binding site on gp120 is only exposed after binding to CD4, thus this site is not accessible for neutralising antibodies.
What are ‘elite controllers’?
The non-progressors; the immune system in some individuals can gerenate neutralising Abs that work against multiple HIV isolates
What is the process of enriching for Abs that recognise the CD4 binding site of HIV gp120?
Create probe that mimics CD4 binding site (RSC3)
Create a variant that has a single aa change within CD4 binding site (ΔRSC3)
Select for Abs that bind RSC3 but not ΔRSC3 - retain CD4 BS recognition
*Abs that bind both are NOT specific for CD4 BS
Large scale isolation of bNt Mabs from memory B cells
- Sort single IgG B cells that bind a modified gp160 trimer called 2CC
- Recover the IgV genes from each cell
- Re-express as a recombinant, complete IgG in a test tube
Strategies to isolate and characterise HIV-specific Abs from HIV-infected individuals
Can use high through sequencing machine - which B cells mount highest response?
Cell sorting methods identifying individual rare B cells that recognise virus by using parts of virus as base to fish them out (structural analyses and epitope mapping)
Did the bNt Mabs come from HIV reactive B-cells?
No. Ab’s ability to neutralise is a property of the mutations that make them specific. Reverting the neutralising Mab VRC01 to germline (H/L) to check binding and neutralisation showed that initial Abs could not bind to virus.
Describe the arms race between HIV and Abs.
Response, escape, counter-response shape the Ab repertoire to HIV.
Abs acquire increasing numbers of SHM in GC.
Rapid escape mutation in Env.
Increasing breadth & potency of neutralisation.
Successive emergence of broadly neutralising Abs in patient that are at first strain specific bu then specific bNAb emerge
Possible late bNAbs do not recognise initiating virus.
Each HIV escape is into an increasingly limited ‘space’.
How can we change the rational design of Anti-HIB Mabs with increased and improved neutralising activity?
Examine binding site at the aa level of Vh and CD4, could make an AA exchange of glycine for an aromatic AA (Y, W, F) greatly improving affinity.
Broadly neutralising Mabs
Mono-therapy was ineffective, tri-therapy is partially effective and penta-therapy is effective.
*Virus is suppressed, but not cleared, hence viral rebound occurs as Abs disappear
Penta-therapy with broadly neutralising Mabs is effective
- shows that can contain HIV with sufficient range of neutralising specificities, which may inform vaccine design
- reveals therapeutic approach to suppress HIV1 viraemia, possibly helping those unable to take HART or in conjunction with HART
- possible for long term production of Abs in people using Adenovirus vectors
- shows what is required for prophylaxis and way of stopping vertical transmission
